Ethylene alpha-olefin (polyethylene) copolymers are typically produced in a low pressure reactor, utilizing, for example, solution, slurry, or gas phase polymerization processes. Polymerization takes place in the presence of catalyst systems such as those employing, for example, a Ziegler-Natta catalyst, a chromium based catalyst, a metallocene catalyst, or combinations thereof.
A number of catalyst compositions containing single site, e.g., metallocene, catalysts have been used to prepare polyethylene copolymers, producing relatively homogeneous copolymers at good polymerization rates. In contrast to traditional Ziegler-Natta catalyst compositions, single site catalyst compositions, such as metallocene catalysts, are catalytic compounds in which each catalyst molecule contains one or only a few polymerization sites.
Many catalyst systems have complex ligands around a catalytic metal site. The ligands are selected to determine the final properties of the polymer formed from the catalyst. For example, different ligands may change the productivity of the catalyst, the polymer chain lengths formed before termination, the rate of incorporation of comonomers, and the like. These ligands may function by modifying the physical environment around the catalytic metal site, for example, increasing the steric hindrance around the catalytic metal site, among others.
Control of these properties is obtained for the most part by the choice of the catalyst system. Thus, the catalyst design is important for producing polymers that are attractive from a commercial standpoint. Accordingly, improved techniques for synthesizing catalyst ligands are desirable.